He hoped the document would find its way into the public domain and expose the US president as a "madman".

The four-page memo recorded April 2004 Oval Office talks between the two leaders on events in the city of Falluja.

Its contents were so secret that much of the trial was held behind closed doors with the press excluded.

The trial centred around allegations that Keogh, a communications officer in the Cabinet Office, had leaked the document to O'Connor, who left a copy in constituency papers for Mr Clarke, the former Labour MP for Northampton South, in May 2004.

Keogh told the jury he wanted it to be used by MPs to ask questions in the House of Commons and also be seen by the 2004 US Democratic presidential candidate, John Kerry.

However, when Mr Clarke found the memo, dated April 16 2004, he called the police.

Keogh was found guilty on two counts of breaking the Official Secrets Act by making a damaging disclosure of part of a government document in his possession as a crown servant without lawful authority.

The jury found O'Connor guilty on a single charge of making a damaging disclosure of a document passed to him illegally, breaching the same Act.

Keogh said he did not believe the publication of the document's contents would harm Britain or its troops abroad, although it would cause embarrassment to Mr Bush.

David Perry QC, prosecuting, said it contained "high-level strategic discussions between world leaders".

"The prosecution say the unauthorised disclosure of information in this case is likely to prejudice the capability of the armed forces either to carry out their tasks or lead to the loss of life or the possibility of loss of life or injury," he said.

The court heard how a record of the meeting, which was held just before the handover of power to the Iraqi authorities, had been taken by Matthew Rycroft, Mr Blair's private secretary for foreign affairs.

The US secretary of state, Colin Powell, and the national security adviser, Condoleezza Rice, were also in attendance.

The document - marked "secret and personal" - was addressed to Geoffrey Adams of the Foreign Office, accompanied by a note that read: "This must not be copied further and must only be seen by those with real need to know."

It was then sent by secure fax to 10 Downing Street, where it was distributed.

Mr Perry went through details of who had seen the memo, including the PM's chief of staff, Jonathan Powell, his foreign policy adviser, David Hill, his official spokesman, Tom Kelly, and the joint intelligence committee.

Many copies were destroyed, and others were handed to police. During the distribution process, the memo arrived at the Cabinet Office communications centre in Whitehall, where Keogh was on duty.

Mr Perry said Keogh then took the faxed copy or a copy of the faxed copy. The court heard that after initially denying his involvement, Keogh had admitted leaking the information.

"His intention was to put the document into the public domain, and that's where Mr O'Connor came on to the scene," he said.

O'Connor admitted copying the document after Keogh told him about it when they met at Northampton Labour club. The researcher told the jury he left the memo for his boss so he would return it to the appropriate authorities.

Rex Tedd QC, defending Keogh, told the judge that the civil servant had not acted for a political motive but had been following his conscience.

"He acted out of conscience. No doubt, he did so misguidedly and he did so in a way which was likely to cause damage," Mr Tedd said. "He did not act out of political motive or financial gain or personal advance."

During legal arguments, it emerged that Mr Blair wrote a letter personally thanking Mr Clarke for the return of the memo.

Sir Nigel Sheinwald, the prime minister's leading foreign policy adviser, said only a small number of people - eight in all - had attended the April 2004 meeting "because we knew it would be a sensitive discussion about Iraq and other matters". He said Mr Bush and Mr Blair had also discussed "military tactics".

Sir Nigel, who said his advice to Mr Blair covered the "waterfront" of foreign, defence and security issues, was persistently questioned about whether documents were marked secret simply to cover up political embarrassment, but denied it.

Mr Tedd added: "The real position, I suggest, is that central to any principle of confidentiality is protecting any American leader from public embarrassment by the disclosure of what is said."

Both men were granted bail and made no comment as they left the court. Sentencing was adjourned until tomorrow.